There are a lot of causes to be hard of hearing or functionally deaf. We are not going into detail but will only highlight the most common causes:

hereditary deafness, the result of genetic mutation and / or identifiable syndromes (thousands currently identified) of which some can be medically treated, passed on by either one or both parents in some

premature births, where knowledge and technical advances reduced the gestation period of survival to such an extend that some organs have not developed to functional maturity

illnesses of the brain e.g. brain tumors, epilepsy etc.

bacterial infestations that affect the brain and / or inner ear e.g. meningitis the most common cause but medically controllable by inoculation and if diagnosed, treatable with antibiotics, although discovered late, damage could be irreversible (most common cause of deafness in African societies).

bacterial infestation that affects the ear (middle ear), curable by medication but if left untreated, could lead to loss of balance and / or tinnitus, both indications of severe infestation

chemical damage e.g. medication, environment etc.

severe mechanical (motor / industrial accidents etc) damage to the brain and / or hearing organ's, where injuries are fatal to the whole or portions of an organ and not able to rehabilitates to functionality